The present invention relates generally to current loop driving circuits and, more particularly, to a current loop drive module with a dynamic compliance voltage.
In general, two wire, loop circuits are used to provide signals for a variety of devices, for example, valve actuators or meters. Loop circuits typically include a current loop driving circuit which varies the current in the loop, generally from 0 to 20 mA, according to a received driving signal. For example, when the loop circuit is used to control a valve actuator, a controller provides a voltage proportional to the current that should be supplied to the controlled actuator. A current loop drive circuit, including a voltage-to-current converter that may contain a current mirror circuit, generates the 0–20 mA signal based on the received voltage.
The voltage supply provided to the current loop drive circuit, commonly referred to as the compliance voltage, is selected such that it is sufficient to drive the current loop signal across the range of expected loads. For example, the loads may range in resistance from ˜750 ohms for a solenoid valve to ˜0 ohms for a panel meter. To provide adequate range, typical loop driving circuits are provided with compliance voltage sources of approximately 24V.
The power dissipated in the current loop system is determined by the current signal and the compliance voltage (i.e., P=0.02 A×24V=0.48 W), not the resistance of the load. In cases where the load has a relatively low resistance, most of the power is dissipated as heat in the current loop drive circuit and not the load. Typical drive modules include circuitry for controlling multiple current loop channels. Due to the potential for significant power dissipation in the drive module, the number of channels a drive module can support for a given volume is limited. Also, the components used in the drive circuitry must be sized appropriately to handle the heat.
Given the restrictions imposed by the power dissipation requirements, it is difficult to reduce the cost per channel and increase the density of the analog outputs. The number of channels provided in a given module is typically limited based on the worst case power dissipation scenario. Alternatively, a user may be provided with guidelines that allow the determination of acceptable loads for a given compliance voltage and the number of loads that may be simultaneously active. Such restrictions limit the range of applications that may be served by a drive module. If a lower compliance voltage is provided, sufficient voltage may not be present to drive the current loop signal over its full range. Increasing the compliance voltage increases the range, but limits the number of channels for a given volume.
Therefore, there is a need for a current loop drive module that has adequate range to drive a variety of load types, but that can reduce the power dissipated in the drive module to allow a higher channel density.
This section of this document is intended to introduce various aspects of art that may be related to various aspects of the present invention described and/or claimed below. This section provides background information to facilitate a better understanding of the various aspects of the present invention. It should be understood that the statements in this section of this document are to be read in this light, and not as admissions of prior art. The present invention is directed to overcoming, or at least reducing the effects of, one or more of the problems set forth above.